Certain methods are known for coupling the output of high-power diode lasers into solid-state lasants. Edge-emitting high-power diode lasers are necessarily broad-area devices or arrays of narrow-width diode lasers because the potential for catastrophic optical damage to the mirrors dictates that the optical outputs be limited typically to 10 to 20 mW per micron of emission stripe width. Typical high-power diode lasers used to pump solid-state lasants include aluminum gallium arsenide (AlGaAs) diode lasers. Examples of such diode lasers include Model No. SDL-2480-P1 with continuous wave (CW) output power of 3.0 watts (W) and an emission width of 500 .mu.m; Model No. SDL-2462-P1 with CW output power of 1.0 W and an emission width of 200 .mu.m; and Model No. SDL-2432-P1 with CW output power of 0.5 W and an emission width of 100 .mu.m, all of which are manufactured by Spectra Diode Labs, 80 Rose Orchard Way, San Jose, Calif. Use of AlGaAs semiconductor diode lasers to optically pump solid-state lasers has led to development of compact, solid-state lasers.
Broad-area lasers are described by Thompson in "A Theory for Filamentation in Semiconductor Lasers", Optoelectronics, 257-310, vol. 4, (1972) and by Kirkby, et al. in "Observations of Self-Focusing in Stripe Geometry Semiconductor Lasers and Development of a Comprehensive Model of Their Operation," IEEE Journal of Quantum Electronics, 705-719, vol. QE-13 (1977). Such broad-area lasers (emission width of typically greater than 5 .mu.m) exhibit a filamentary structure in their optical near-field patterns. The filament structures arise from a nonlinear interaction between the carriers and the optical field in the active area of the laser. The process of stimulated emission effectively reduces the gain profile within the active area and results in an increase in the refractive index in the portion of the active area contributing most strongly to the optical mode. This region of increased refractive index is bounded by regions of the active area that do not contribute so strongly to the optical mode and are characterized by smaller refractive index values. This lateral variation in refractive index in a local region within the active area of the diode laser can form a local lateral index guide.
When the active area is broader than about 5-10 .mu.m, as is the case in typical high-power laser diodes used for solid-state laser pumping, several, or in some cases, many such index-guided regions may form. Stimulated emission within each such lateral index-guided region within the active area may occur in the form of a filament that is only partly spatially coherent or is spatially incoherent with respect to neighboring filaments. This filamentation phenomena is, therefore, a fundamental source of lateral spatial incoherence in high-power laser diodes and, consequently, places limits on the optical brightness obtainable from such devices.
Although such high-power broad area diode lasers have with varying degrees of efficiency been used to optically pump solid-state lasers and to produce useful solid-state laser output at a variety of emission wavelengths, improved methods for coupling the optical output from diode lasers into solid-state lasants are highly desirable.
A method for theoretically obtaining high-power, nearly diffraction-limited optical output from a high-power diode laser has recently been described by Tilton, . . . DeFreez, et al., in "High Power, Nearly Diffraction-Limited Output from a Semiconductor Laser with an Unstable Resonator," IEEE Journal of Quantum Electronics, 2098-2108, vol. 27, No. 9, (September 1991). The high-power AlGaAs diode laser described therein demonstrates high power (greater than 1 watt from both facets) and nearly diffraction-limited optical output. The reference states that "[f] or many semiconductor laser applications such as solid-state laser end pumping . . . , single-lobed, diffraction limited beams of hundreds of milliwatts are required." Coupling the optical output from such an unstable resonator into a solid-state laser is described in U.S. Pat. No. 5,260,963 of Baird and DeFreez for Method and Apparatus for Efficient Operation of a Solid-State Laser Optically Pumped by an Unstable Resonator Semiconductor Laser, which is assigned to assignee of the present invention and is herein incorporated by reference.
Many important laser applications require laser operation at visible or ultraviolet wavelengths. Moreover, a compact source of coherent visible or ultraviolet light output suitable for use in hospital operating rooms and similar medical environments is also highly desirable for use in a wide range of medical treatments, such as photo-activation of therapeutic drugs.
A variety of methods have been described for generating laser output in the 400 nm to 600 nm wavelength range from solid-state lasers and diode lasers by utilizing the nonlinear process of second-harmonic generation (SHG). For example, several methods have been described for producing SHG laser output in the 520-540 nm wavelength range from diode-pumped, solid-state lasers containing a neodymium-doped lasant. Baer, et al. in U.S. Pat. No. 4,653,056 describe one such method in which an AlGaAs diode laser end-pumps a solid-state laser resonator containing a neodymium-doped yttrium aluminum garnet (Nd:YAG) rod and potassium titanium phosphate (KTP) nonlinear crystal to produce SHG laser output at 532 nm. As described in "Second Harmonic and Sum-Frequency Generation to 4950 and 4589 A.degree. in KTP," IEEE Journal of Quantum Electronics, vol. QE-24, No. 1 (January 1988), such bulk KTP crystals are phase-matchable for type-2 second-harmonic generation down to 495 nm. To generate SHG wavelengths shorter than 495 nm, other bulk nonlinear materials are required.
Kozlovsky, et al. in "Efficient Second Harmonic Generation of a Diode-Laser-Pumped CW Nd:YAG Laser Using Monolithic MgO:LiNbO.sub.3 External Resonant Cavities," IEEE Journal of Quantum Electronics, vol. 24, No. 6 (June 1988), describe producing about 30 mW of SHG output at 532 nm by using a diode-pumped Nd:YAG, single-mode ring laser operating at 1064 nm to pump an external monolithic cavity of nonlinear magnesium oxide:lithium niobate (MgO:LiNbO.sub.3).
Another method of producing SHG laser output at 532 nm is described by Schutz, et al. in "Miniature Self-Frequency-Doubling CW Nd:YAB Laser Pumped by a Diode-Laser," in Optics Communications, vol. 77, No. 2, 3 (15 Jun. 1990). Schutz, et al. describe producing a SHG output of about 10 mW at 532 nm by end-pumping a laser resonator containing the self-frequency-doubling lasant neodymium:yttrium aluminum boron (Nd:YAB) with 870 mW emitted by a AlGaAs diode laser array operating at an output wavelength of 807 nm.
Risk and Lenth in "Room-Temperature, Continuous-Wave, 946-nm Nd:YAG Laser Pumped by Laser-Diode Arrays and Intracavity Frequency Doubling to 473 nm," Optics Letters, Vol. 12, No. 12 (December 1987), describe a method to pump a 1 mm length rod of Nd:YAG with two 0.25 W diode laser arrays whose output are combined using a polarizing beamsplitter cube arrangement. The method employs a 5 mm long crystal of lithium iodate (LiIO.sub.3) cut for Type I phased-matched frequency doubling of 946 nm output at room temperature in the solid-state laser resonator cavity and produces approximately 100 .mu.W of SHG blue light at 473 nm. Risk, Pon, and Lenth in "Diode Laser Pumped Blue-Light Source at 473 nm Using Intracavity Frequency Doubling of a 946 nm Nd:YAG Laser," Applied Physics Letters, vol 54, No. 17 (24 Apr. 1989), describe further work on a similar method employing a single 0.5 W laser diode to end-pump a solid-state laser resonator containing a 1 mm long Nd:YAG rod and a 3.7 mm long potassium niobate (KNbO.sub.3) nonlinear crystal to produce. 3.1 mW of blue output at 473 nm.
Methods have also been described in which the laser output from diode lasers are directly frequency doubled. Kozlovsky, et al. describes such a method in "Generation of 41 mW of Blue Radiation by Frequency Doubling of a GaAlAs Diode Laser," Applied Physics Letters, vol. 56, No. 23, (4 Jun. 1990). They employ a monolithic ring resonator of KNbO.sub.3 to convert 105 mW of incident diode laser power at 856 nm to 41 mW of blue 428 nm output power. The room-temperature wavelength limit for non-critical phase-matching in KNbO.sub.3 of about 860 nm is likely to prevent SHG wavelengths significantly shorter than the wavelengths they describe from being produced using KNbO.sub.3 and similar methods.
In "Blue Second Harmonic Generation in KTP, LiNbO.sub.3 and LiTaO.sub.3 Waveguides," Phillips Journal of Research, vol. 46, 231-265 (1992), Jongerius, et al. describe conversion of an infrared pump beam into a blue beam by SHG through coupling the pump beam into channel waveguides that have been diffused into the surface of KTP, LiNbO.sub.3, or lithium tantalate (LiTaO.sub.3) substrates. They describe achieving 6 mW of 460 nm output power from a periodically segmented domain-inverted KTP waveguide by pumping the waveguide with 920 nm output from a Ti:Sapphire solid-state laser.
It would be desirable to find a method to produce a compact, diode-pumped solid-state laser that can produce higher power visible or ultraviolet laser output in the 350-550 nm range using the nonlinear process of SHG. For example, improved methods for coupling the optical output of high-power diode lasers, especially those having improved lateral spatial coherence such as an unstable resonator semiconductor laser, into the mode volumes of a solid-state lasant such as Cr:LiCAlF or Cr:LiSAF are highly desirable. Such methods for pumping CR:LiCAlF or CR:LiSAF to ultimately produce usable frequency-doubled optical output in the 360-460 nm range are described in detail in copending U.S. patent application Ser. No. 07/873,408 of Baird and DeFreez for High Power, Compact, Diode-Pumped, Turnable Laser, which is assigned to assignee of the present invention and herein incorporated by reference.